Comparative study of photocatalysis with bulk and nanosheet graphitic carbon nitrides enhanced with silver

The main goal of this research is to investigate the effectiveness of graphitic carbon nitride (g-C3N4, g-CN) in both bulk and nanosheet forms, which have been surface-modified with silver nanoparticles (Ag NPs), as photocatalysts for the degradation of acid orange 7 (AO7), a model dye. The photodegradation of AO7 dye molecules in water was used to test the potential photocatalytic properties of these powder materials under two different lamps with wavelengths of 368 nm (UV light) and 420 nm (VIS light). To produce Ag NPs (Ag content 0.5, 1.5, and 3 wt%) on the g-CN materials, a new synthesis route based on a wet and low-temperature method was proposed, eliminating the need for reducing agents. The photodegradation activity of the samples increased with increasing silver content, with the best photocatalytic performances achieved for bulk g-CN samples and nanosheet silver-modified samples (with the highest content of 3 wt% Ag) under UV light, i.e., more than 75% and 78%, respectively. The VIS-induced photocatalytic activity of both examined series was higher than that of UV. The highest activities of 92% and 98% were achieved for the 1.5% Ag-modified g-CN bulk and nanosheet materials. This research presents an innovative, affordable, and environmentally friendly chemical approach to synthesizing photocatalysts that can be used for degrading organic pollutants in wastewater treatment.


Synthesis of g-C 3 N 4 -bulk and nanosheet materials and their surface modification with Ag nanoparticles
The bulk g-C 3 N 4 powder was prepared by heating melamine according to the work published elsewhere 36 .Melamine (3 g) was placed in a covered alumina crucible and calcined in a muffle furnace at 620 °C for 2 h and then at 550 °C for 1 h (heating rate: 15 °C/min) and allowed to cool freely in the furnace to room temperature.The sample was ground in a porcelain mortar to obtain a fine powder and labelled as g-CN-JP.Later, the as-prepared g-CN-JP powder (0.5 g) was placed in a covered alumina crucible and heated at 500 °C for 3 h (heating rate: 10 °C/min) in a muffle furnace, and the thermal exfoliation process was done.Upon the thermal exfoliation process of g-CN bulk (g-CN-JP), the resulting g-CN nanosheet material was received.The material was ground in a porcelain mortar to obtain a fine powder and labelled as g-CN-exf.Throughout the article, the terms g-CN bulk and g-CN-JP, as well as g-CN nanosheet and g-CN-exf, are used interchangeably.Both g-CN bulk and nanosheet materials were surface-modified with silver nanoparticles.The first stage involved dissolving pure silver nitrate (AgNO 3 , Lachema) in a 96% ethanol (EtOH) solution.The stoichiometric amount of silver nitrate used in the synthesis was 0.005, 0,015, and 0.03 g.In the second stage, 1 g of g-CN-JP or g-CN-exf powders were added to the as-prepared AgNO 3 -EtOH-H 2 O solutions.After forming a yellow homogeneously dispersed suspension, the mixtures were magnetically stirred for several hours.The yellowish suspensions were then air-dried for a few hours at 70 °C and then at 150 °C.The prepared samples were labelled as g-CN-JP-x% Ag and g-CN-exf-x% Ag, where x indicates the amount of Ag (0.5, 1.5, and 3.0 wt%) in the final composites.The composites were ground in a porcelain mortar to yield a fine powder.The samples labelled g-CN-JP-150 and g-CN-exf-150 represented materials obtained in the proposed low-temperature chemical synthesis based on their processing in ethanol solution only (without silver).The synthesis flowchart of silver-modified g-CN bulk and nanosheet powders is illustrated in Fig. 1.
The Fourier-transform infrared spectroscopy (FTIR) spectra were taken by a Nicolett™ Summit FT-IR spectrometer (Nicolet Instrument Company) with attenuated total reflection (ATR) detection mode using ZnSe-based ATR accessories.The spectra were collected in the range 4000-400 cm −1 with a resolution of 4 cm −1 and 16 scans.No additional sample preparation procedure was applied before the measurements.
Raman microscopic measurements were made using a Thermo Scientific, Waltham, MA, USA, DXR Raman microscope equipped with a 780 nm solid-state diode-pumped (DPSS) laser source.Exposure time was 1 s, and the number of exposures was 50 during data collection at each measurement.Gratings of 400 lines/mm and a 50 µm slit confocal aperture setting with a 10X objective was used.The spectral resolution was ~ 2 cm −1 in each case.
A Shimadzu Ltd.UV-2600 Series spectrometer with a 60 mm diameter Shimadzu Ltd.IRS-2600Plus integrating sphere was employed to evaluate the UV-Vis DRS spectra of dry powder samples at room temperature between 220 and 800 nm.A reference sample of powder BaSO 4 was utilized.The reflectance data were transformed using the Kubelka-Munk (K.-M.) function, and the indirect bandgap energies (E g ) values were determined using Tauc's plot.
The photoluminescence (PL) emission spectra of all derived g-C 3 N 4 -based compounds were assessed at room temperature using an Edinburgh Instrument Ltd FLSP920 Series spectrometer in the wavelength range of 350-600 nm.The spectrometer used a 450 W non-ozone xenon lamp (Steady state Xe900 lamp) and an R928P detector (PMT detector).In this case, the Czerny-Turner configuration was used.The absorption spectra were measured using an Edinburgh Instruments, Ltd.FLSP920 Series spectrometer.
The morphology of the powder samples was investigated using a JEOL JSM-7610F Plus scanning electron microscope (SEM) (Tokyo, Japan) with a Schottky cathode at 15 keV in a high vacuum chamber.Powder samples were prepared on brass stubs with carbon tape and coated with a 13 nm nanometric layer of gold.Elemental composition analysis and mapping were performed using the energy-dispersive X-ray spectroscope (EDS) AZtec Ultima Max 65 (Oxford Instruments, Abingdon, UK).
The TEM images were recorded in a Jeol JEM-1400plus type microscope (Japan) at 120 keV.Prior to measurement, one drop of the ethanol-dispersed sample was introduced onto a Formvar foil film-covered copper grid and left to dry.The JMicroVision software was used to determine the average particle diameter of the silver NPs on the g-CN material.

Electrochemical investigations
10 mg of each sample was dispersed in 5 ml of deionized water and then ultrasonically homogenized for 30 min.Of this suspension, 30 μl was dropped onto the surface of a glassy carbon electrode, and then this assembly was dried at 85 °C for 3 h.Electrochemical tests were performed on a Metrohm Autolab PGSTAT302 potentiometer, with a glassy carbon electrode (GCE), Ag/AgCl (3M KCl), and a Pt sheet serving as the working, reference, and counter electrode.Before taking each measurement, the electrode surface was cleaned with a polishing set for solid-state electrodes.An aqueous solution of 0.1 M KCl was used as an electrolyte, which was bubbled with nitrogen for 30 min before electrochemical measurement.The Mott-Schottky recordings were taken at an AC frequency of 300 Hz with an amplitude of 10 mV.

Photocatalytic activity measurements
The photocatalytic activity of all g-C 3 N 4 -based materials was determined by degrading acid orange 7 (AO7) dye.Under UV (368 nm) and VIS (420 nm) light, photodegradation of an aqueous solution of AO7 at a concentration of 7.14 × 10 −4 mol/l was conducted.In a typical procedure, 50 mg of catalyst was dispersed in a glass beaker in deionized water and magnetically stirred at 300 rpm at room temperature to obtain a homogeneously dispersed suspension.After that, AO7 was added to the suspension.All as-prepared suspensions were magnetically stirred at 300 rpm for 1 h at room temperature in dark conditions to estimate the adsorption and desorption equilibrium of AO7 dye on the catalyst.Afterwards, a UV lamp of wavelength 368 nm or VIS lamp of wavelength 420 nm was switched on for ultraviolet or visible light irradiation measurements.During the specified experimental time, two millilitres of a uniformly dispersed suspension of the test material in demineralized water with AO7 dye were collected using a syringe.The collection was done in the dark or under UV/VIS light.Afterwards, the suspensions were filtered using a syringe filter with a pore size of 0.45 µm (CHROMAFIL GF/RC-20/25 filters, Macherey-Nagel, Germany).The absorbances of the filtered suspensions were measured at 480 nm to separate the photocatalytic material.The degradation of AO7 was monitored in a 1 cm diameter quartz glass microcuvette in the presence of demineralized water using a Helios Epsilon spectrometer set at 480 nm.For all studied samples, the percentage photocatalytic activity degradation of AO7 (PA sample ) was calculated according to the equation: PA sample = (1 − (C eq.AO7 /C 0 AO7 )) × 100%, where C eq. AO7 is the equilibrium state of AO7 concentration, and C 0 AO7 is AO7's initial concentration.
The silver-modified g-CN nanosheet (thermally exfoliated) samples (Fig. 2B), in contrast to the bulk samples, are visibly less ordered (Fig. 2A), as confirmed by full width at half maximum (FWHM) evaluation of the most intensive peak d = 0.3 nm of the g-C 3 N 4 phase.The width of nanosheet (thermally exfoliated) samples (Fig. 2B) is more significant than that of non-exfoliated samples (Fig. 2A).As the amount of silver content increases, there is a corresponding increase in the width observed.This is another phenomenon that has been noted.The most disordered material was the nanosheet (thermally exfoliated) sample with 3 wt% of silver (g-CN-exf-3%Ag).It should be noted that the peak of the silver phase is difficult to detect as its amount and crystalline state are very low.

Spectroscopy (FTIR, Raman, DRS, PL) studies
The IR spectra of both analyzed sample series are shown in Fig. 3A,C.The recordings show characteristic sharp absorption peaks at 811, 1233, 1318, 1403, 1463, 1563, and 1645 cm −1 and broad absorption peaks in the region 3000-3700 cm −1 due to N-H hydrogen bonds and -OH bonds.
The sharp band at 811 cm −1 was assigned to the breathing mode of out-of-plane vibrations of triazine units.The very strong absorption band at 1233 cm −1 and strong band at 1318 cm −1 , 1403 cm −1 correspond to the C-N stretching vibrations (in which the C-NH-C and C-C vibrations are probably involved).The band at 1463 cm −1 corresponds to the number of rings of the heterocyclic structure, and the strong bands at 1563 cm −1 and 1645 cm −1 indicate stretching vibrations of the C=N bonds of g-C 3 N 4 heterocycles 62,63 .The wide absorption peaks observed between 3000 -3700 cm −1 consist of several components.Among them, bands at 3100 cm −1 and www.nature.com/scientificreports/3300 cm −1 can be attributed to hydrogen bonds, while those at 3350 cm −1 can be assigned to -OH water groups.
The remaining components of this broad band can be assigned to the N-H stretching of the remaining amino groups 64 .The analyzed IR spectra were similar for silver-modified g-CN bulk and nanosheet series.This may be due to the detection limit of the method, with no additional bands corresponding to Ag content occurring.
The Raman spectra of all investigated samples are presented in Fig. 3B,D.Three characteristic peaks of graphitic carbon nitride at 470 cm −1 , 710 cm −1 , and 1235 cm −1 can be recognized.In addition, Raman shift is recorded in the measured spectrum range 400-2000 cm −1 .These bands are assigned to the N-C-N stretching vibrations, bending vibrations = C (sp 2 ), and the vibrations of C-N bonds in the heterocycle of g-C 3 N 4 65 , as confirmed by FTIR studies.It is worth noting that there were no observable changes in the Raman spectra after the silver modification of both bulk and nanosheet g-CN materials.However, the impact of the exfoliation process from bulk to nanosheet g-CN material is evident as the intensity of the Raman spectral bands in the non-exfoliated series showed a significant decrease compared to the exfoliated series (as shown in Fig. 3B,D).
The optical properties of the silver-modified g-CN bulk and nanosheet materials were evaluated using diffuse reflectance spectroscopy (Fig. 4A-D).The Tauc method was used to generate a Tauc plot by analyzing the reflectance spectra of all samples (as shown in Fig. 4C,D).This method involves transforming the UV-Vis reflectance spectrum into a modified Kubelka-Munk function, plotting it against photon energy (i.e., hv, where h represents the Planck constant and v represents the photon's frequency), and determining the E g value by extrapolating the slope to zero.
The band gap energies (E g ) for silver-modified g-CN bulk and nanosheet materials were 2.66-2.69eV and ca.1.70 eV, respectively.The results of the calculated E g are presented in Table 1.It should be noted that the silver content ranged only from 0.5 to 3 wt%.Therefore, the effect of increasing Ag concentration on the surface of g-CN in both series of materials, and thus the obtained results in E g values, is negligible.It can be observed that both g-CN-JP-x%Ag and g-CN-exf-x%Ag series show an increase in E g value with an increase in Ag content.The samples belonging to the g-CN-exf-x%Ag and g-CN-JP-x%Ag series, which have been modified with Ag, exhibit a notable rise in absorption in the visible light range spanning from 400 to 800 nm (see Fig. 4A,B).This growth in absorption is due to the Surface Plasmon Resonance (SPR) phenomenon caused by the Ag nanoparticle effect 26,37,66,67 .The emission peak observed at 472 nm for g-CN-JP and g-CN-JP-150 can be attributed to electron transitions between antibonding π* and bonding π states and between π* and lone pairs of electron states 68,69 .As mentioned in this study 70 , nitrogen loss of C 3 N 4 was observed because of its annealing.
Furthermore, the optical properties were measured using photoluminescence (PL) spectra analysis at room temperature.The results are depicted in Fig. 4E,F.Both series of studied materials showed decreasing PL intensity with increasing Ag content.The silver-modified g-CN bulk and nanosheet series exhibit a red shift in the PL maxima (λ max ) from 472 to 479 nm and 464 to 468 nm, respectively.Additionally, this shift in emission peaks is accompanied by a decrease in the PL emission bands, as presented in Table 1 2,37,71 .
The absorption spectra of the silver-modified g-CN bulk and nanosheet materials were measured, and the estimated results are included in the Supplementary Material in Fig. S1A,B.The absorption spectrum of g-CN-JP (Fig. S1A) displays an absorption edge at approximately 470 nm, which corresponds well with the band gap energy (E g = 2.66 eV, as shown in Table 1), as reported by Cao et al. 72 and Cao and Yu 13 .The absorption edges for other composites (g-CN-JP-x% Ag) shown in Fig. S1A are located within the bandgap energy range of about 470-480 nm.Fig. S1B shows the visible absorption edges of pristine materials g-CN-exf, g-CN-exf-150, and their composites (g-CN-exf-x% Ag) in the range of 463-470 nm 13,72 .The optical band absorption edges are depicted  www.nature.com/scientificreports/for g-CN-exf, g-CN-exf-150, and their composites (g-CN-exf-x% Ag), and they are shifted towards higher wavelengths.This shift is causing a decrease in the band gap energies compared to pristine g-CN-JP, g-CN-JP-150, and their composites (g-CN-JP-x% Ag), respectively.The increase in the bond length of the sp 2 C-N clusters during the thermal exfoliation process may be due to the extension of the polymeric network of g-C 3 N 4 by more linking tris-triazine units, as illustrated in Fig. S1A,B.This phenomenon is a typical feature of thermal exfoliation 73 and agrees with the band gap energies (see Table 1).

Morphology (SEM, TEM) examination
The energy dispersion microanalysis (EDS) method was used to confirm the presence of silver in samples with the highest content (3 wt%), see Fig. 5.Samples were coated with gold (10 nm thick layer) to avoid charging during observation in an electron microscope chamber.The measured values for both samples g-CN bulk and nanosheet-are basically the same and around the detection limit-0.3-0.5% wt.For samples containing 0.5 and 1.5 wt% of silver, this method may not detect it.It should also be noted that the quantification of light elements such as C, N, and O is biased and has a higher standard deviation when using EDS microanalysis.However, this method is suitable for confirming the presence of Ag in the sample with its highest content and complements the results of other analyses.Additionally, the SEM images (secondary electrons regime) of all silver-modified g-CN bulk and nanosheet materials at two magnifications: 2500× (scale bar 10 µm) and 17000x (scale bar 1 µm) are provided in Supplementary Material in Fig. S2.
The TEM images of the samples indicate a similar structure to that presented in the SEM images.Contrary to the EDS's detection limit, silver nanoparticles can be detected on the g-CN-JP materials (Fig. 6), and their increasing amount can also be observed as the Ag content increases.
For verification, the following figure (Fig. 7) clearly shows the presence of Ag nanoparticles as the Ag content increases.The particle size distribution was also analyzed, and a decrease in the average diameter for the  Ag nanoparticles, from 8.1 ± 1.9 nm to 5.2 ± 1.6 nm, was observed as the Ag content varied from 0.5 to 3 wt%, respectively.
The presence of Ag is proved based on the EDS results; however, they are not clearly/unquestionably distinguished on the "exfoliated" nanosheets except for the sample with 3% Ag content, indicating further differences between the systems.One possible explanation is that the NPs are smaller in the nanosheets.

Electrochemical measurements
To further investigate the efficiency of the photocatalyst, Mott-Schottky measurements were performed.This technique relies on applying a bias voltage to modulate the width of the space-charge interfacial double layer and, therefore, the value of capacitance when the solution side of the interface can be approximated by the Helmholtz model of the double layer 74 .The Mott-Schottky Eq. ( 1) describes the relation between the space charge capacitance C and the applied potential V. where ε r is the dielectric constant, ε o is the dielectric constant of vacuum, N D is the dopant density for n-type semiconductor, T is the temperature, K is the Boltzmann constant and e is the elementary electron charge.This equation is applicable in the linear part of the Mott-Schottky plot.
The values of flat band potentials (V FB ) were calculated as the intercepts of the extrapolation lines with the potential axis in the Mott-Schottky plot, which is the function of the inverse of the measured capacitance squared on the potential (Fig. 8).
For n-type semiconductors, it is assumed that flat band potential is nearly identical to the conduction band (V CB ) edge potential according to Eq. ( 2) 75,76 .
(1)  where V FB(NHE) is the flat band potential recalculated against NHE electrode at pH = 7, ∆V is the potential of Ag/ AgCl reference electrode vs NHE.The valence band edge potential (V VB ) was calculated using Eq. ( 3) 75,76 .
where E g is the band gap energy.
When the light interacts with the photocatalysts, holes are formed in the valence band, whereas electrons are transferred to the conduction band.The reactions concerning the formation of •O − 2 and •OH can be ascribed as follows (Eqs.4-6).
The CB, VB edge potentials and band gap energies are given in Fig. 9.For all samples, the energy value of the CB edge potential is higher than − 0.33 V, indicating that only superoxide radicals are formed after irradiation.Since the position of the valence band edge potential is much lower than the standard redox potential •OH − / • OH , the formation of hydroxyl radicals is improbable.The shift of V CB to more negative values and deviating from the standard redox potential of •O 2 / • O 2 pair indicates an increase in the ability of the conduc- tion band electrons of the given samples to form superoxide radicals ( •O − 2 ), and this ability was highest for the samples containing 1.5 and 3 wt% Ag, both in the bulk and nanosheet series.

Degradation of AO7 under UV or VIS light
The photocatalytic degradation of the AO7 using silver-modified g-CN bulk and nanosheet photocatalysts was examined under UV (368 nm) and VIS (420 nm) light.The results are presented in Figs. 10 and 11 and Table 2. Further, during all photodegradation processes using UV/VIS lamps, the photolysis process of the AO7 dye was also performed, and the results are shown in Supplementary Material in Fig. S3.These findings demonstrated that AO7's stability under UV/VIS light irradiation did not cause a significant change in its absorbance (see Fig. S2). Figure 10A-D shows that the photocatalytic performance of the pristine g-CN bulk material (sample g-CN-JP) was limited compared to the g-CN bulk material prepared in EtOH-H 2 O solution and heat-treated at 150°C in the air (sample g-CN-JP-150).The most noticeable difference between the g-CN-JP (38%) and g-CN-JP-150 (67%) samples is observed during the 3 h of the photodegradation process of AO7 under visible light (refer to Fig. 10C,D).The photocatalytic properties are enhanced by a small amount of Ag (only 0.5 wt%), observed within 15 min after turning on the UV or VIS lamp.The effect of silver-modified samples within only 0.5 wt% was observed in our previous work when the composite melem/g-C 3 N 4 served as a main photocatalytic material 26 .The photocatalytic activities of analyzed samples under UV and VIS lamps increase due to the increasing Ag content.This indicates that the added Ag NPs had a considerable influence on the photocatalytic behavior of g-CN bulk material, and the surface modification succeeded in obtaining g-CN-JP-x% Ag composite photocatalysts.For the silver-modified g-CN bulk materials, the photodegradation activities under UV and VIS light were between 45-75% and 38-92%, respectively.The highest photodegradation activities (after 3 h of irradiation) were achieved for the sample with the 1.5 and 3 wt% Ag.A study by Alenazi and co-authors 77,78 presented  www.nature.com/scientificreports/ a similar observation for the surface modification of g-C 3 N 4 material with Mo 6+ and W 6+ in the composites Mo 6+ @g-C 3 N 4 and W 6+ @g-C 3 N 4 , respectively, at concentrations between 0.5 and 3 wt%.The Mo 6+ @g-C 3 N 4 composites were tested to remove the potential pollutant 2,4-dichlorophenoxyacetic acid under natural sunlight 77 .The W 6+ @g-C 3 N 4 composites were tested for removing chlorophenol derivatives in natural sunlight exposure 78 .
The works mentioned clearly indicate that modifying g-C 3 N 4 in an amount of 0.5-3 wt% is sufficient to achieve the desired effects, making the process cost-effective.It can also be argued that the photocatalytic activity of bulk g-CN surface-modified with Ag nanoparticles is satisfactory under visible light due to suppressing the recombination of photogenerated electron/hole pairs.Considering the g-CN nanosheet materials (g-CN-exf, g-CN-exf-150) (Fig. 11), it is evident that the photodegradation activities achieved under UV/VIS light are higher than those of the pristine g-CN bulk samples (g-CN-JP, g-CN-JP-150) (Fig. 10).These results generally indicate the efficiency of the thermal exfoliation process carried out for the g-CN bulk material.After 3 h of UV to VIS irradiation, the difference in photocatalytic activity for unmodified g-CN nanosheet materials over AO7 was 35%.The thermal exfoliation process increases dye molecules' adsorption capacity, leading to a better ability to suppress the recombination of charge carriers 73,[79][80][81] .Under UV and VIS light, the photodegradation activities for the nanosheet silver-modified g-CN materials ranged from 51 to 78 and 89-98%.The VIS-induced photocatalytic activity of both examined series was higher than that of UV.It is also clearly visible that the highest activities were achieved for the silver-modified g-CN nanosheet material.The Ag NPs that were used to modify the surface of g-CN bulk and nanosheet materials acted as photoinduced electron collectors, allowing them to be segregated from holes, as demonstrated in Fig. 4, which resulted in a decrease in PL intensity as well as an increase in the photocatalytic degradation processes towards AO7 dye.The silver-modified g-CN nanosheet material showed the highest activity (Fig. 11, Table 2).Supplementary Material includes a literature survey of composites of graphitic carbon nitride (g-C 3 N 4 , g-CN) with silver nanoparticles that were utilized for photodegradation processes involving selected dyes, such as methyl orange (MO), methylene blue (MB), rhodamine B (RhB), and acid orange 7 (AO7).This survey is presented in Table S1.The presented results demonstrate the effectiveness of utilizing metallic silver nanoparticles to modify the surface of a graphitic carbon nitride material.Our work focuses on the cost-effective and straightforward approach that we employed to modify the surface of g-CN bulk and nanosheet materials containing silver nanoparticles at concentrations of 0.5, 1.5, and 3 wt% without the use of a reducing agent.The resulting materials were then investigated as photocatalysts in wastewater treatment towards acid orange 7 dye under UV and VIS irradiation.
The difference in degradation rates between the silver-modified g-CN bulk and nanosheet series under UV and VIS light is also depicted in Figs. 10 and 11, and this finding is further supported by estimated kinetic constants (Table 2).The pseudo-first-order reaction equation 26,77,78 ln(C/C 0 ) = k cal t, where C 0 is the initial concentration of AO7, C is the concentration of AO7 at the time t of the measurement, and k is the kinetic constant, was used to derive the calculated kinetic constants (k cal t) for all materials.The obtained kinetic constants for silver-modified g-CN bulk materials were 4.4-8.8× 10 −3 min −1 (under UV lamp) and 2.7-10.5 × 10 −3 min −1 (under VIS lamp).For the silver-modified g-CN nanosheet materials series, the determined kinetic constants under UV and VIS light were in the ranges 5.1-8.7 × 10 −3 min −1 and 7.9-16.2× 10 −3 min −1 .It can be observed that the parameters of the kinetic constants for both series of samples increase with increasing Ag content, and this phenomenon was observed at both wavelengths of light.The results indicate that the Ag content greatly influences the photodegrading rate (k cal ) of the silver-modified g-CN bulk and nanosheet samples.The highest photodegrading rate was estimated for all silver-modified g-CN bulk and nanosheet materials.Under UV light, both silver-modified g-CN series showed similar values of kinetic constants in the range 7.0-8.8× 10 −3 min −1 .Under VIS irradiation, the silver-modified g-CN series exhibit kinetic constants in the range of 9.5-16.2× 10 −3 min −1 .The g-CN-exf-1.5%Ag sample achieved the highest kinetic constant values (16.2 × 10 −3 min −1 ) and considerable photodegradation activity (98%) after 3 h of exposure to visible light towards AO7.
The enhanced photocatalytic performance of g-CN bulk and nanosheet materials modified with Ag NPs towards AO7 under UV and VIS light can be explained as follows.Various studies have reported that •O 2 − is the dominant reactive species during photocatalytic reactions under UV/VIS for pristine materials based on g-CN-JP, g-CN-JP-150, g-CN-exf, and g-CN-exf-150 26,82,83 .The addition of silver (g-CN-exf-x% Ag and g-CN-JP-x% Ag)

Figure 1 .
Figure 1.Flowchart of the preparation of silver-modified g-CN bulk and nanosheet powders.

Figure 8 .
Figure 8. Mott-Schottky plots at 300 Hz for silver-modified g-CN bulk (A) and nanosheet (B) materials.The flat band potential values are shown in the plots.

Figure 9 .Figure 10 .
Figure 9. Energy diagrams of CB, VB edge potentials and energy gaps for silver-modified g-CN bulk (yellow) and nanosheet (green) samples.

Figure 11 .
Figure 11.Photocatalytic degradation C/C 0 and activity processes of silver-modified g-CN nanosheet materials.Comparison of the UV lamp: 368 nm (A, B) vs. VIS lamp: 420 nm (C, D).

Table 1 .
Indirect band gap energy (E g ) values (Kubelka-Munk function, Tauc spectra) from DRS, maximum emission bands from photoluminescence spectra of silver-modified g-CN bulk and nanosheet materials.

Table 2 .
Activity and kinetic constant (after 3 h) of silver-modified g-CN bulk and nanosheet materials.Comparison of the UV (360 nm) vs. VIS (420) nm lamps.